There is little information about the electrophysiologic properties of the conduction system of newborn and young animals, and changes that occur as the animal matures. It has been observed that the transmembrane resting and action potential characteristics of tissue from fetal and neonatal hearts differ from those of adult cardiac tissue in many species. In general, resting membrane potential appears more positive in cardiac tissue obtained from young hearts than from those of adults. Neonates have also been observed to respond differently to such pharmacologic agents as cardiac glycosides. It is proposed to study the electrophysiologic properties of the maturing canine heart from neonate to the adult. Specifically, we will examine, using ion-selective liquid ion exchanger and conventional microelectrode techniques, the relationship between the potassium equilibrium potential (EK) and the membrane potential (Em) in normal neonatal and adult cardiac Purkinje fibers. Such information is important to the understanding of how the mechanisms responsible for development and maintenance of transmembrane ionic and voltage differences change with age. The effects of ouabain on the relationship between EK and Em and other action potential characteristics will be examined. This information may elucidate the mechanisms by which neonatal Purkinje fibers respond differently from adult fibers to cardiac glycosides. In addition, direct measurement of intracellular potassium and sodium ion activities under conditions that have been suggested to induce marked electrogenic sodium pumping will allow assessment of the degree and importance of this phenomenon in Purkinje fibers of animals in different stages of development. This type of analysis may provide an indirect means of assessing the ouabain sensitivity of the sodium-potassium activated adenosine triphosphatase (ATPase) from different aged tissues.